Electrodeposition of platinum metals



Patented Aug. 21, 1934 ELECTRODEPOSITION OF PLATINUM TALS Edmund M. Wise, Wcstficld, N. J. asslgnor to The International Nickel Company,

a corporation of Delaware Application June 20, 1932,

York, N. Y.,

No Drawing.

Inc., New

Serial No. 618,303

13 Claims.

The present invention relates to the electrodeposition of metals of the platinum group and more particularly to a process of electrodepositing metals of the platinum group continuously and uniformly on a commercial scale.

It is well-known that heretofore a variety of processes and baths have been proposed and used for the electrodeposition of metals of the platinum group. Generally speaking, these prior baths contained a phosphate or other acid group ammine or ammino platinum metal compounds. In addition, baths containing compounds of the platinate character have also been proposed. These baths have had certain limitations and shortcomings due to the fact that they were subjeoted to variations in performance during use,

and also to the fact that only relatively thin plates could be produced on a practical scale. The resulting plates were frequently not highly lustrous and frequently tended to be rather dark in color and sometimes were cracked and porous. Furthermore, some of these baths were open to the objection that the replacement or maintenance of the platinum metal content was troublesome because the addition of platinum metal in the form of chloride tends to cause an accumulation of undesirable salts in the bath which requires, in many instances, the discarding of the solution after a relatively short time of operation. The investigators in this art have been searching for a simple, economical and practical process and bath for the plating of metals of the platinum group which can be carried out on a practical and commercial scale and which avoids the shortcomings and disadvantages of'prior baths and processes. As far as I am aware, the art has not been provided with a satisfactory and successful bath or process for depositing these platinum group metals.

I have discovered a process anda plating bath which overcomes the disadvantages noted hereinabove and which is satisfactory when carried out on a commercial scale.

It is an object of the invention to provide a process for the electrodeposition of metals of the platinum group including platinum, palladium, iridium, rhodium, and the like and which can be operated in a continuous manner to give consistent and uniform results.

It is another object of the invention to. provide a process for the electrodeposition of metals of .the platinum group for the production not only of thin, brilliant and lustrous plates, but also for the production of relatively thick plates.

It lsa further object of the invention to provide a bath for the electrodeposition of platinum metals which can be maintained in good operating condition in a relatively simple manner and without much trouble.

It is also within the contemplation of the invention to provide a bath'for the plating of platinum metals in which certain of the com- 'ponents are regulated and maintained within a critical range throughout the operation of the bath.

Other objects and advantages of the invention will become apparent from the following description.

For the purpose of giving those skilled in the art a better understanding of the invention,- a preferred procedure for carrying the invention into practice will be set forth. It is to be understood, however, that the description of the preferred embodiment of the invention is merely given for the purposes of illustration and is not to be taken as a limitation.

In general, it has been found that aqueous solutions of soluble platino, irido, rhodo, and pallado nitrites of metals of the alkali group or of the alkaline earth group constitute satisfactory baths for the electrodeposition of platinum metals. It is preferred to employ sodium salts of the foregoing type although other soluble alkali and alkaline earth complex platinum metal nitrites may be employed. As a result of experiments, it has been found that alloys containing various proportions of platinum metals can be electrodeposited from baths containing a plurality of platinum metals in the form of double nitrites of the character mentioned hereinabove.

The new plating baths may be made by dissolving the selected complex nitrite, such as platinum I or palladium tetranitrite, or iridium or rhodium hexa-nitrite, in water. Of course, the aforesaid double nitrites may be produced in situ, thus utilizing the product resulting from the reaction between the selected platinum metal salt and the appropriate amount of alkali or alkaline earth nitrite in an aqueous solution which is slightly acid and warm.

' The new plating bath may be prepared in any appropriate manner as one skilled in the art will readily understand. For instance, a bath for plating platinum may be prepared from platinum sulfate and sodium nitrite. This bath is prepared in the following manner: To a solution containing 10 grams of platinum (as sulfate) there was added 14.2 grams of. sodium nitrite. This caused a slight evolution of nitrous fumes. The solution was heated to the boiling when it changed from red to lemon yellow. The salt imparting this lemonyellow color to the solution is the desired plating salt, sodium platino tetranitrite. There is also some sodium sulfate formed.

PtSOa-i-NaNOz PtSO4+2NaNOz+Ba(NO2)2 NazPt (NQa) 4+Na2SO4 NazPMNOz) 4+BaSO4 The complex nitrite'is stable and may be obtained in crystallized form by evaporation and crystallization. Other complex nitrites such as NazPd(NO2)4, Na Rh(NOz)s, and NaaIr(N02)s, may be prepared.

It has been found that it is essential to regulate and maintain a certain platinum metal ion concentration by maintaining special and critical ratios of nitrite to platinum metal. It has been discovered that the molecular ratio of nitrite to palladium or platinum should be regulated and maintained throughout the operation of the bath at about 4 to 1 while the molecular ratio of nitrite to rhodium or iridium should be about 6 to 1. By regulating and maintaining the aforesaid critical ratios, it has been found that the plating baths can be used continuously to give consistent and uniform plates of the platinummetals without defects occurring in the plates or interruptions in the plating operations.

In general, the concentration of the platinum metal in the electrolyte may be varied considerably, the lower limit being iixed by the cathode current density to be employed in plating, while the upper limit is set either by the solubility of the salt or by the cost of the bath. I have found that a platinum metal content of between about 2 and about 15 g/l, gives satisfactory results and I have found that the preferred range of platinum metal or about 4 and 8 g/l.

It has been found that the new plating baths operate best with a controlled pH value which should lie within a range of about 1 to about 10. For best results in the production of bright blushfree plates, it has been found the pH value of about 4 to about 8 is necessary for palladium and that the pH value ranging from about 6 to about 7 is most satisfactory. Rhodium baths have been found to be operative over a considerable range of pH and to give good results when in approximately neutral condition, that is, with a pH value of about '1. In regard to iridium baths, it has been found that the best results can be obtained with baths having a pH value of ap proximately 1. Platinum may be successfully deposited over a pH range of from about 1, to 7 but a pH value of about 6 to about 7 is most satisfactory. As a result of many experiments, it

has been found that the cathode current efil-' ciency is influenced by the pH value of the bath. In general, if the pH values deviate materially from the ranges given hereinabove the cathode current efliciency is substantially decreased. For instance, platinum baths operated at a temperature of about 95 C. with pH values of 3.0, 6.8, and 7.4 gave cathode current efliciencies of 16.8%, 19% and 13.4% respectively. With palladium baths operated at-a temperature of about 5.0". C. with pH values of 1, 4.4, and 6.2, gave cathode current efliciencies of 55%, 82%, and 83% respectively.

Palladium and rhodium can be deposited at room temperature, although more satisfactorily at elevated temperatures. The deposition of platinum requires a warm bath, whilst iridium also requires a warm bath. For example, we find that the best results are obtained with palladium when the bath is operated at from about 40 C. to about 60 C. while platinum is best deposited from baths operated at from about 50 C. to about C.

It has been found that with the complex nitrite baths with the special ratio of nitrite to platinum metal, the presence of buffer reagents, such as a sodium phosphate, sodium acetate or acetic acid, or mixtures thereof, renders it relatively easy to maintain the baths at the desired pH value. Soluble, more or lss neutral conducting salts, such as sodium sulfate, increase the conductivity of the bath and have been found to be useful additions for improving the throwing power of this type of bath. Furthermore, it is preferred to have the new baths substantially free from halides which tend to impair the stability of baths operated at relatively high pHs (much above 7.2) and at high temperatures.

Anodes are preferably made of one of the ductile platinum metals, although carbon or graphite may be used.

Good deposits may be obtained directly on precious metal cathodes and upon the usual high copper content alloys. Nickel is best plated by giving it a preliminary strike with a metal possessing a lower solution pressure than nickel. The strike seems particularly necessary where palladium plates are required. A palladium plating solution in which the palladium ion concentration is materially lower than that characteristic of the complex palladium nitrite forms a very suitable strike.

The permissible cathode current density tends to increase with the concentration of platinum metal or metals in the bath, with the operating temperature, degree of agitation, hydrogen ion concentration, and within limits, with the platinum metal ion concentration. With palladium, good plates have been obtained at cathode current densities up to about 8 ampere/square foot, with platinum up to about 16 ampere/square foot and with rhodium or iridium up to about 40 ampere/square foot.

The platinum metal content of the bath may conveniently be maintained by introducing from time to time the requisite quantities of the corresponding platinum metal in the form of the corresponding complex nitrite, to replace the platinum metal which has been consumed as a result of the operation of the bath.

It is to be noted that the electroplates obtained from prior plating baths were usually blushed, i. e'., they exhibited a blue haze or milkiness when viewed at an angle to the incident light. It was necessary to remove the blush by polishing. This operation wasted precious metal and removed the protecting precious metal from elevated portions and surfaces of the plated object. In contrast to prior plates, the present invention provides electroplates which are free from blush and do not require a finishing operation after plating. As one skilled in the art understands, the latter are very important advantages.

-It is possible to plate platinum metal alloys from baths containing the desired platinum metals in the form of nitrites. For example, a bath containing Pt 8 grams/liter and Pd 4 grams/ liter when operated at a temperature of about 55 C. and a current density of 2 amperes/square foot give a platinum-palladium alloy plate con taining 7.68% platinum and 92.32% palladium.

Although the present invention has been described with respect to certain specific reagents, temperatures, current densities and the like, it is to be understood that variations and modifications may be made within the scope of the invention as those skilled in the art will readily understand.

I claim:-

1. The process of electrodepositing a metalof the platinum group which comprises regulating the ratio of nitrite to platinum metal in the electro-platlng bath containing a soluble complex nitrite of a metal of the group consisting of sodium, potassium, lithium, calcium, barium, strontium and magnesium and a platinum metal in a range of about 4:1 to about 6:1, making the article to be plated the cathode of said electroplating bath, passing current through said bath to cause the deposition of platinum metal on said article and maintaining the aforesaid ratio in said bath throughout the electro-plating.

2. The process of electrodepositing a metal of the platinum group which comprises regulating the ratio of nitrite to platinum metal in the electro-plating bath containing a soluble complex nitrite of a metal of the group consisting of sodium, potassium, lithium, calcium, barium, strontium and magnesium and a platinum metal in a range of about 4:1 to about 6:1, adjusting the pH value in said bath from about 1 to about 10, making the article to be plated the cathode of said electro-plating bath, passing current through said bath to cause the deposition of platinum metal on said article and maintaining the aforesaid ratio in said bath throughout the electroplating.

' 3. The process of electrodepositing a metal of the platinum group which comprises regulating the ratio of nitrite to platinum metalin the electro-plating bath containing. a soluble complex nitrite of a metal of the group consisting of sodium, potassium, lithium, calcium, barium, strontium and magnesium and a platinum metal in a range of about 4:1 to about 6: 1, adjusting the pH value of said bath to a value not in excess of about 7.5, maintaining the said bath in a warm condition,'making the article to be plated the cathode of said electro-plating bath, passing electric current through said bath to cause the deposition of platinum metal on said article and maintaining the aforesaid ratio in said bath throughout the electro-plating.

4. The process of electrodepositing a metal of the platinum group which comprises regulating the ration of nitrite to platinum metal in the electro-plating bath containing a soluble complex nitrite of a metal of the group consisting of sodium, potassium, lithium, calcium, barium, strontium and magnesium and a platinum metal in a range of about 4:1- to about 6:1, controlling the platinum metal from about 2 grams/liter to about 15 grams/liter, making the article to be plated the cathode of said electro-plating bath, passing current through said bath to cause the deposition of platinum metal on said article and maintaining the aforesaid ratio in said bath throughout the electro-plating.

5. The process of electrodepositing a metal of the platinum group which comprises regulating the ratio of nitrite to platinum metal in the electro-plating bath containing a soluble complex nitrite of a metal of the group consisting of sodium, potassium, lithium, calcium, barium,

strontium and magnesium and a platinum metal in a range of about 4:1 to about 6:1, controlling the platinum metal from about 4 grams/liter to about '8 grams/liter, making the article to be plated the cathode of said electro-plating bath, passing current through said bath to cause the deposition of platinum metal on said article and maintaining the aforesaid ratio in said bath throughout the electro-plating.

6. The process of electrodepositing a metal of the platinum group which comprises regulating the ratio of nitrite to platinum metal in the electro-plating bath containing a soluble complex nitrite of ametal of the group consisting of sodium, potassium, lithiumgigalc ium, barium, strontium and magnesium and platinum metal in a range of about 4:1 to about 6:1, maintaining the electro-plating bath in a heated condition at a temperature of about 40 C. to about (3., making the article to be plated the cathode of said current through said bath to cause the deposition of platinum metal on said article and maintaining the aforesaid ratio in said bath throughout the electro-plating.

7. The process of electrodepositing a metal of the platinum group which comprises regulating the ratio of nitrite to platinum metal in the electro-plating bath containing a soluble complex nitrite of a metal of the group consisting of sodium, potassium, lithium, calcium, barium, strontium and magnesium and a platinum metal in a range of about 4:1 to about 6:1, making the article to be plated the cathode of said current through said bath to cause the deposition of platinum metal on said article, adjusting the cathode current density from an effective value up to about 40 amperes per square foot, and maintaining the aforesaid ratio in said bath throughout the electro-plating.

8. The process of electrodepositing a metal of the platinum group which comprises regulating the ratio of nitrite to platinum metal in the electro-plating bath containing a soluble complex nitrite of a metal of the group consisting of sodium, potassium, lithium, calcium, barium, strontium and magnesium and a platinum metal in a range of about 4:1 to 6:1, making the article to be plated the cathode of said electro-plating bath, and an anode of the group comprising ductile platinum metal and graphite, passing electric current through said bath to cause the deposition of platinum metal on saidarticle and maintaining the aforesaid ratio in said bath throughout the electro-plating.

9. The process of electrodepositing a metal of the platinum group which comprises regulating the ratio of nitrite to platinum metal in the electro-plating bath containing a soluble complex nitrite of a metal of the group consisting of sodium, potassium, lithium, calcium, barium, strontium and magnesium and a platinum metal in a range of about 4:1 to about 6: 1, providing a soluble neutral conducting salt in said bath, making the article to be plated the cathode of said electro-plating bath, and passing electric current through said bath to cause the deposition of platinum metal on said article and maintaining the aforesaid ratio in said bath throughout the electro-plating.

10. A'bath for electrodepositing a metal of the platinum group comprising an aqueous solution of a complex nitrite of a platinum metal and of a metal of the group consisting of sodium, potasemme 12. A bath for electrodepositing palladium comprising an aqueous solution of a palladium tetra. nitrite of the group comprising sodium and potassium. I

13 A bath for electrodepositing palladium con-f1.

taining an aqueous solution of a palladium tetra nitrite of the group consisting of sodium and potassium and having a pH valve .of about 6 to about 8.

EDMUND M. WISE. 

